(1) Field of the Invention
The present invention relates to a cylindrical secondary battery such as a nickel-hydrogen secondary battery, and particularly to a structure of a current collector plate joined to one of cores that extends from an upper part of an electrode assembly and a current collector lead used for connecting the electrode assembly with a sealing cover.
(2) Related Art
Generally, a cylindrical secondary battery such as a nickel-hydrogen secondary battery is used for an electric source of an HEV (Hybrid Electric Vehicle) and a PEV (Pure Electric Vehicle). For such a use purpose, there has been a demand for increasing the output efficiency of the cylindrical secondary battery and reducing the electrical resistance within the battery.
In response to such a demand, Japanese Patent Application Publication No. 2004-235036 for example proposes a positive current collector lead that is inserted between a positive current collector plate and a sealing cover and is used for electrically connecting therebetween. The following describes an art disclosed in Japanese Patent Application Publication No. 2004-235036, with reference to FIG. 1, FIG. 2A, and FIG. 2B.
As shown in FIG. 1, a cylindrical secondary battery 901 has the structure in which an outer casing can 937 having a cylinder shape with a closed bottom houses therein an electrode assembly 930 that has been spirally wound, an opening end edge 937b is caulked such that a sealing cover 938 is provided at an opening of the outer casing can 937. An insulating gasket 939 is inserted between the sealing cover 938 and an internal surface of the outer casing can 937.
The electrode assembly 930 is composed of a positive electrode plate 931 and a negative electrode plate 932 with a separator 933 inserted therebetween. An upper part and a lower part of the electrode assembly 930 are welded and joined to a positive current collector plate 935 and a negative current collector plate 934 in the Z-axis direction, respectively. The negative current collector plate 934 is welded and joined to a can bottom 937c of the outer casing can 937. The positive current collector plate 935 is joined to an inner bottom surface of a sealing plate 938a of the sealing cover 938 via a positive current collector lead 910. Note that the operations of the positive current collector plate 935 are controlled by inserting a vibration-proof ring 940 between an inner surface of a sidewall of the outer casing can 937 and an inner surface of a reduced-diameter part 937a. 
The sealing cover 938 is composed of the sealing plate 938a and a positive electrode cap 938b, and a valve plate 938c and a spring 938d that are housed in an internal space formed by the sealing plate 938a and the positive electrode cap 938b. 
As shown in FIG. 2A, the positive current collector lead 910 is a single metal plate that has been bent so as to have a cylinder shape, and has a cylinder top part 911 joined to the sealing plate 938a and a cylinder bottom part 912 joined to the positive current collector plate 935. Also, an opening part 911a is formed in the cylinder top part 911 for inserting a welding electrode at a time when the negative current collector plate 934 is welded to the can bottom 937c of the outer casing can 937. Furthermore, projections 911b and 912b are respectively provided in the cylinder top part 911 and the cylinder bottom part 912, as welding points.
As shown in FIG. 2B, the cylinder bottom part 912 of the positive current collector lead 910 is welded and joined to the positive current collector plate 935 at the projections 912b as welding points. Note that the positive current collector plate 935 has provided therein a plurality of burring holes 935a for joining to a core of the positive electrode plate 931 of the electrode assembly 930 and slits 935b for protecting the eddy current.
According to the cylindrical secondary battery 901 proposed in Japanese Patent Application Publication No. 2004-235036, as shown in FIG. 1, the positive current collector plate 935 is connected with the sealing plate 938a via the positive current collector lead 910 having a cylinder shape. Accordingly, the electrical resistance is reduced and as a result the output loss can be reduced, compared with the connection configuration using a lead plate having a rectangle shape.
However, there has been a demand for further reduction in the electrical resistance of a cylindrical secondary battery. The cylindrical secondary battery 901 proposed in Japanese Patent Application Publication No. 2004-235036 does not sufficiently satisfy such a demand, and accordingly further improvement is necessary.
Here, according to the cylindrical secondary battery 901 proposed in Japanese Patent Application Publication No. 2004-235036, there is a limited space formed between the positive current collector plate 935 and the sealing cover 938 in the outer casing can 937. Accordingly, it is substantially difficult to broaden a cylinder width of the positive current collector lead 910. Also, there is a method to be considered, for increasing a plate thickness of the positive current collector lead 910. However, if this method is adopted, the rigidity of the positive current collector lead 910 is increased, and it is difficult to reduce the overall height of the battery. As a result, a problem will occur such as the reduction in the sealing performance or the energy density of the battery.
Especially, in the case of a cylindrical secondary battery for HEV or PEV, after the positive current collector plate 935 is welded and joined to the sealing plate 938a and the positive current collector lead 910, the sealing cover 938 is pressed toward the can bottom 937c of the outer casing can 937 so as to reduce the overall height of the battery. However, if the thickness of the positive current collector lead 910 is increased, it is difficult to perform such a press process.